Healthy reef landscapes can help restore degraded coral reefs

Healthy reefs are known as vibrant homes for colorful corals and fish. Like any active ecosystem, they have their own sounds and can be quite noisy. The grunts, crackles and noises of the fish and crustaceans that live there and the sound of healthy coral growth can echo through the water. Larval animals may use some of this sound to help them determine where to put down roots or when it is time to grow. Broadcasting these healthy reef sounds can encourage coral larvae to recolonize degraded or damaged coral reefs. The findings are detailed in a study published March 13 in the journal Royal Society for Open Science.

A shot to calm down

As adults, corals are immobile. Their larval stage is their only chance to move and find that perfect habitat. They float or move with the currents to find the right conditions to settle and then anchor to the seabed. Previous studies have shown that chemical and light cues can help influence this decision, but this new work examined the role that sound may play. They can certainly sense these vibrations, as corals do not have traditional ears.

[Related: Google is inviting citizen scientists to its underwater listening room.]

“What we’re showing is that you can actively induce coral establishment by playing sounds,” Nadège Aoki, a co-author of the study and a doctoral candidate at the Woods Hole Oceanographic Institution (WHOI), said in a statement. “You can go to a reef that’s degraded in some way and add the sounds of biological activity from a healthy reef, potentially helping this really important step in the coral life cycle.”

Soundscapes of rocks

To get a closer look, a team of researchers conducted experiments in the US Virgin Islands in June and July 2022. They collected larvae from a hard coral species called Porites astreoides. It is more commonly known as mustard hill coral, due to its yellow color and bumpy shape. They distributed the larvae along three reefs along the south coast of St. John. Of these reefs, Tektite is relatively healthy. Cocoloba and Salt Pond are more degraded, having fewer fish and less coral cover.

mustard hill coral in the florida keys.  it is rich yellow in color and grows in mounds.
Mustard Hill Coral that was replanted at Carysfort Reef in the Florida Keys. CREDIT: Greg McFall/NOAA

The team installed an underwater speaker system on the Salt Pond reef and placed larval cups at distances of 3.2, 16.2, 32.8, and 98.4 feet from the speakers. For three nights, they then played healthy rock sounds at Salt Pond that were recorded at Tektite in 2013. They also set up similar installations at Tektite and Cocoloba, but did not play any of the recorded rock sounds.

After collecting the bowls, they found that many more coral larvae had settled in the bowls at Salt Pond than the other two reefs. Larvae settled there an average of 1.7 times more in sound-enriched environments than in those that were not. Cups that were about 16 feet from the speakers saw the highest rates of larval settlement, but even cups that were almost 100 feet away had more larvae settled at the bottom than those where no sounds were played.

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